66KV to 132KV Cables Tests requirement

Following tests as Shown in figure must be done for 66KV & 132KV Cables as per IS: 60840 and IS:7098 part 3.

You May visit at link for difference between XPLE and PVC Cables:-

http://electrialstandards.blogspot.in/2014/03/difference-between-xlpe-and-pvc-cables.html

Also find out advantages of XLPE cables over Oil filled Paper Insulated cables:-
http://electricalsystembasics.com/2014/04/xlpe-cables-advantages-oil-filled-paper-insulated-cables.html

As per IS Standards 7098 Part 3 :-
Cross-Linked Polyethylene Insulated Thermoplastic Sheathed Cables - Specification. Part -3 for working voltages from 66 Kv up to and including 220 kV.

is Copper as earth pit is better then GI earth pit?

It has been often found that most of think that material used for earthing will reduce earthing resistance.

It has been often misconception that copper earthing plate or rod will have lower earth pit resistance then GI earthing plate or rod.

Lets clear this fundamental regarding as often falling in this trap will lead to higher investment on earthing by using copper as earthing material.

If one considers a plate electrode, the approximate resistance to earth is:

Resistance formula for Pipe earthing

(R) = (100r/2πL) X loge (4L/d)

·          L= Length of Pipe/Rod in cm

• d=Diameter of Pipe/Rod in cm
• r = Resistivity of Soil Ohm-meter.

Resistance formula for Plate earthing

(R) = (r/A) X under root(π/A).

·         r = Resistivity of Soil Ohm-meter.

·         A=Area of Earthing Plate m3.

As can be seen from the above formulas, only the resistivity of the soil and the physical dimensions of the electrode play a major role in determining the electrode resistance to earth.

The material resistivity is not considered anywhere in the above formulas.

Hence, irrespective of the material of construction of the earth electrode, any material of given dimensions would offer the same resistance to earth.

Material will help in the sizing and number of the earthing conductor or the protective conductor. Because, the short time withstand current ratings do vary according to the material of construction of the conductor.

So will never be losing end even when you are spending more money on Copper earthing instead of GI earthing.

Difference between grounding and earthing

There is always misconception related to difference between grounding and Earthing.

Lets discuss the same:-

It’s generally Myth about ‘Grounding’ is that grounding done for current carrying parts to ground like transformer or generator neutral and ‘Earthing’ means connection of non-current carrying parts to ground, like metallic enclosures.

But as per IS 3043-1987  the terms ‘earthing’ and ‘grounding’ are synonymous. 

Possibly the various nomenclature is because of the usual conflicting using British language between your People in America & the British. 

As the British called it as being ‘earthing’, the People in America called it as being ‘grounding’. 

IEC & IS Standards refer as ‘earthing’, while IEEE & ANSI Standards refer as ‘grounding’.

To quote from IEC 60050:

“The terms ‘earth’ in addition to ‘ground’ have both been generally use to explain the most popular energy/signal reference interchangeably all over the world in the Electro-technical terminology. As the USA and other United States nations favor using the term ‘ground’, European nations such as the United kingdom and lots of other Eastern nations like the term‘earth’”.

The IEEE meaning of grounding is: “Ground (ground system) -

A performing connection, whether intentional or accidental, through which an electrical circuit or equipment is attached to the earth or some performing body of relatively large extent that serves  instead of the earth.”

To be used within Europe, when the generally recognized terms were changed as below, then your meaning continues to be same.

“Earth (earth system) - A performing connection, whether intentional or accidental, through which an electric circuit or devices are attached to the mass of earth or some performing body of relatively large extent that serves instead of the mass of Earth.”

Fault current direction

There are very misconceptions about the direction of fault current.

Usually it has been misinterpreted that as from source to load as everyone thinks that’s source will supply load current and voltage.

Fault current always flows backwards i.e. if fault occurred at Motor then first it's O/L get tripped not directly causing Transformer tripping. Although current is drawn from Source but fault current always from Fault end to backward, as during fault at load heavy current will flows at load end as during fault load resistance reduced to very low value . That is how Electrical system is designed for load fault current.

Electrical system should be designed in Such a way that protection equipment connected for protection of load much immediately isolate load from source.

e.g. if load rated current is 300A then load end MCCB/SFU must be of 400A with fault current rating depending upon Transformer rating and impedance. Now Load end MCCB is of 400A and protection equipment such be set at 300A or less then 300A so that if any fault occurred load must get disconnected as soon as current reached 300A level and Short circuit , Earth fault settings done accordingly.

If we are designing system for 160KW motor having rated current of 297A then we must select 315A as its main MCCB/SFU and O/L Relay setting done at 297A or less then 297A , Earth fault relay setting should be done at 10% and 400A MCCB/SFU will be used as main Source for whole electrical system of 160W motor.

Star Delta Starter Changing Direction of motor

Star Delta Starter connected motor direction can be changed by changing either connections at main input line connections and O/P connections at Main and Delta contactors.

Star-Delta Starter Connection changing circuit is shown above.

Induction motors not kept running in Star Connections

Induction motors are not kept on running in Star connections due to following reasons:-

In Star connections, Phase current= Line current and

Phase voltage= Line voltage/ √3.

As Power = √3X Line Voltage XLine Current X Power Factor

This means that during starting voltage get reduced by factor 1/√3.

So current is also get reduced by factor 1/√3.

So Torque also gets reduced by factor 1/3 as Torque is directly proportional to square of voltage.

Due to reduced voltage Motor unable to take-up higher loads.

So motor power get reduced by 1/3 times then in Delta connections that is why motor is not kept running in Star connections. As motor is get under utilized.
So Motor will kept on running at reduced power and means motor will run at low efficiency

Star Delta Starter and applications

Motors up-to 7.5 KW are started using direct online starters but above 7.5 KW induction motors are started using Star-Delta Starters as if motors above 7.5 KW are started directly then there will be voltage disturbances in line due to large starting current surges. So let’s discuss how this Star-Delta Starter works.

Working Principal of Star-Delta Starter:

First of all you must know what Star and Delta connections are:-

Star Connection or Wye Connections:-

In Star connections, 

Phase current= Line current and

Phase voltage= Line voltage/ √3.

This means that during starting voltage get reduced by factor 1/√3.

So Torque also gets reduced by factor 1/3 as Torque is directly proportional to square of voltage.

Current also get reduced by 1/√3 in induction motor if motor is Started in Star connections.

If You want to know about Direct online starter first then click link here

Delta Connections:-

In Delta connections

Phase Voltage = Line Voltage and 

Phase current= Line Current/ √3.

After Starting motor in star connections then transition is done from Star connections to Delta connections after certain delay.

Star-delta Starter Consists following components:

1)     Main, star and delta contactorsà 3 No’s

2)     Timer Star-Deltaà 1 No.

3)     Overload relayà 1No.

4)     Main MCCB or Fuse Unit

Power Circuit of Star Delta Starter:

Main Power circuit of Star-Delta Starter is as Shown below:-

Add caption

In this Circuit there are three power contactors  C1 for Main C2 for Delta and C3 for Star.

Control Circuit of Star-Delta Starter (Open Transition):

Control circuit for Star-Delta Starter is as shown below:-

In that circuit when Start PB is pressed Star and Main contactor gets holds and motor get started in Start connections and after certain delay which was set in timer star contactor get disengage and delta contactor comes in line.

Star and Delta contactor NC’S are take in line both in Delta and contactor line so that only 1 contactor either star or Delta may comes in line.

In Star Delta Starter since Six leads goes at Motor end so in each lead phase current will flow so Overload relay used for Star-Delta circuit is usually for phase current not line current.

There will be always question arises that why Induction motor kept running in Star then visit link below:-

http://electrialstandards.blogspot.com/2014/04/induction-motors-not-kept-running-in.html

 Application of Star-Delta Starter:

In Most of industries for motors above 7.5 KW Star-Delta Starters are used. Star Delta Starters are used as during starting start current get reduced by 1/3 times and also torque get reduced by 1/3 times.

They have low maintenance cost compared to VFD or Soft-Starters which will be discussed later.